1. Field of the Invention
This invention relates to variable focus lenses and more particularly to elastically deformable lenses wherein the optical power of the lens can be varied by small changes of its equatorial diameter.
2. Brief Description of the Prior Art
Variable focus lenses and lens systems have come to be extensively used because they provide convenient solutions to problems which frequently arise in applied optics. Optical systems incorporating such lenses can, for example, provide a focused image of objects at varying distances from the lens on an image plane without changing the distance between the lens and the image plane. They can also be used in optical systems that provide varying magnification without changing lenses.
A number of types of variable focus lenses have been devised. However, the design most widely used at present in optical instruments such as cameras, telescopes, binoculars and microscopes is a multi-element lens wherein the focal length is varied by changing the internal spacing of one or more of the elements along the optical axis.
Another class of variable focus lenses relies on changes in the refracting power of a single lens element effected by changes in the curvature of the refracting surfaces or the refractive index of the lens material.
One type of such single-element variable focus lenses uses a fluid-filled chamber formed by flexible membranes whose curvature can be varied. U.S. Pat. No. 1,269,422, to Gordon, discloses spectacle lenses comprising a pair of optical surfaces formed, e.g., of thin glass, joined at their peripheries to form a chamber which is filled with a transparent liquid. Each lens is mounted in a rim which can be made smaller by tightening a tangent screw to decrease the circumference of the rim. Such tightening is disclosed to increase the refractive power of the lens.
Another design for a variable focus lens uses a fluid-filled chamber with flexible walls wherein the curvature of the walls can be varied by adjusting the amount of fluid contained in the chamber. Such lenses can involve a simple balloon or bladder made of transparent material inflated with a liquid of an appropriate refractive index. Other structures having flexible refracting surfaces whose curvature is adjusted by varying the volume and/or pressure of a fluid within the lens body have also been devised. Lenses of this type are disclosed, for example, in U.S. Pat. No. 3,598,479, to Wright, and U.S. Pat. No. 4,913,536, to Barnea.
Other variable focus lenses use elastically deformable materials that are deformed by various surrounding structures in order to vary the curvature of the optical surfaces. Such lenses are disclosed e.g., in U.S. Pat. No. 4,783,155, to Imataki et al.; U.S. Pat. No. 4,784,479, to Ikemori; U.S. Pat. No. 4,802,746, to Baba et al.; and U.S. Pat. No. 4,859,041, to Suda.
An elastically deformable lens of variable focal length is also disclosed in U.S. Pat. No. 4,444,471, to Ford et al. Ford discloses changing the focal length of an elastomeric biconvex lens by radially stretching the lens by a substantial amount so that the curvature of the optical surfaces is reduced and the refracting power of the lens is thereby also reduced. However, Ford does not disclose or discuss the changes in optical power that occur in an elastomeric lens when it is radially stretched by only a few percent of its diameter.
U.S. Pat. No. 4,712,882, to Baba et al., discloses a variable focus lens comprising a transparent elastic cylindrical body having a radially varying index of refraction wherein the optical power is reduced by radially expanding the cylinder, thereby placing the lens under radial tension. The radial expansion of the lens is accomplished by a piezoelectric element surrounding the cylindrical body of the lens and bonded thereto. Baba discloses that such radial expansion also reduces the positive curvature of refracting optical surfaces at the ends of the cylinder or induces increased negative curvature. In Baba's variable focus lens the radial expansion is conducted generally uniformly along the entire axis of the cylinder.
Other methods for producing a variable focus lens have involved controlling the refractive index of the material from which the lens is made. For example, in a lens formed from a liquid crystal, a varying electric current across the liquid crystal lens can produce a lens of variable power. Other crystals, whose index of refraction can be continuously varied by electrical or mechanical means, can also be used in the manufacture of variable focus lenses.
These previous methods of constructing a variable focus lens have certain deficiencies particular to each of the techniques. For example, moving lens elements within a multielement lens system requires relatively large, heavy and precisely constructed mechanical lens cells, tracks and linkages. In lenses made with material having a variable refractive index the size has had to be limited in order to maintain adequate optical clarity. Any hitherto disclosed variable focus lenses using a fluid-filled balloon or bladder have required a reservoir and means for moving fluid into and out of the lens, which introduces impractical complications. Furthermore, in order to produce continuously variable lenticular astigmatism, the known lenses must be tilted or have unusual shapes.
Accordingly, a need has continued to exist for a method to produce a variable focus lens in which the spherical and astigmatic optical power of the lens can be altered without the need for large mechanical movements, unusual shapes, changes in the index of refraction of the material, or the use of a balloon lens with a reservoir.